The present invention relates generally to valve actuators and, more particularly, to a valve actuator having a novel way to translate a linear motion and force to a rotary motion and force, applicable to rotary actuators for rotary valves.
Rotary actuators for rotary valves such as ball, butterfly, plug, and sector valves, usually provide a 90° turn to the valve shaft, from the closed position to the open position and back. Some valve forms, notably multi-port valves, require 120° and 180° turning. The actuators are generally divided into three categories, namely electric, pneumatic, and hydraulic. Pneumatic and hydraulic actuators are referred to as fluid power actuators. The most common types consist of a linear power source in the form of a fluid power cylinder and piston, plus a device to translate from linear motion to rotary motion.
There are three principle mechanisms used to convert from linear to rotary motion, namely a rack and pinion gear set, and a device known as a scotch yoke, and a simple link and lever. The rack and pinion gear and the scotch yoke have become dominant in the field of valve actuation. These mechanisms, however, have inherent limitations and properties that cause problems in their application, and leave them less than ideal.
The rack and pinion device requires a restraining bearing behind the rack to keep it from jumping off the teeth of the pinion gear. This bearing introduces considerable friction and “sticktion,” and there is backlash and lost motion between the teeth of the rack and the pinion gear, amplified greatly by any wear of the restraining bearing.
The scotch yoke has a nonlinear torque output with respect to position, which requires sizing to the lowest torque output, typically about 60% of maximum, adding cost when applying these actuators. The nonlinear torque output is undesirable in some applications. Under load, the changing torque output can cause acceleration of stroking in the increasing torque direction, and deceleration in the decreasing torque direction, which is why this style of actuator, in the larger sizes, has a reputation for lurching as it strokes. Its mechanism causes a large sideways force on the cylinder's piston rod, requiring lateral bearing support, at some cost. The piston rod flexes due to the lateral force, making this mechanism prone to hysteresis and lost motion, as well as adding to the influences that cause the lurching.
The link and lever mechanism has so many limitations in its application, including nonlinear torque output as in the case of the scotch yoke, that it has been almost completely replaced by the rack and pinion and scotch yoke mechanisms.
A fourth mechanism exists for converting linear motion to rotary motion, but it is used only in some special applications, not as a rotary valve actuator. It consists of a double-ended fluid power cylinder, a loop of chain fastened at each end to the two ends of the piston rods, and two sprocket wheels, one for the torque output shaft and one as a simple idler, all as shown in FIG. 4. Compared to valve actuators, it has relatively high cost, awkward size, and extra friction from the extra wheel. The size is awkward because the unit must have a long dimension of more than three times the length of the cylinder plus one sprocket wheel diameter. The subject invention differs in that it uses a single-ended cylinder, has a heavy plate and double wrapped tangent wheels, resulting in the subject invention having only one rotating component and one set of bearings, not two of each, and it employs different chains for each direction, and it can alternatively employ a plate, wheel, and silent thin metal straps.